The present invention relates generally to antennas, and more specifically the invention pertains to a double-folded monopole for use on a linear antenna to produce a traveling-wave distribution of current.
Probably the most widely used antenna for both transmission and reception is a monopole above a ground plane. It may be used either alone or as an element of an array from kilohertz frequencies up through the microwave band. It is a simple and very inexpensive antenna. It radiates (or receives) a vertically polarized field. It provides 360.degree. coverage in azimuth; elevation coverage is limited by the size and the conductivity of the ground plane. For an infinite, perfectly conducting ground plane the elevation pattern of the monopole has a peak along the horizon. As the ground plane becomes smaller the elevation angle of the peak field rises; there is always a null in the zenith direction.
One of the main limitations of the monopole is that it is a relatively narrow band antenna; that is its input impedance and directional properties are a strong function of frequency. The bandwidth of this type antenna can be significantly increased by placing a resistance of suitable magnitude one-quarter wavelength from its end point. This was called the "Traveling-Wave Linear Antenna" since it has a traveling wave distribution of current up to the resistor.
The main disadvantage of the resistive-loaded traveling-wave antenna is that it is only about 50% efficient because part of the input power is absorbed by the resistor. The double-folded monopole evolved from this traveling-wave linear antenna. The approach was to replace the resistor with a resonant antenna which has a radiation resistance that is approximately equal to that of the matching resistor. Thus the inserted antenna would be driven by the other antenna. The input section will still have a traveling-wave distribution of current up to the inserted element, as before, but now the power that was previously dissipated in the resistor is also radiated. There are many types of antennas that can be used in place of matching resistor. The only constraint is that if the traveling-wave distribution of current on the input section is to be preserved, then the radiation resistance of the inserted antenna should be approximately 240 ohms and the element should be placed one-quarter wave length from the end of the monopole. Another option is to insert antenna elements at distances of odd multiples of quarter wavelengths from the end of the monopole. For this arrangement the radiation resistances have to be adjusted accordingly.
As mentioned above, it has been shown that a traveling-wave distribution of current can be produced on a linear antenna by inserting a resistance of suitable magnitude one-quarter wavelength from its end. The electrical energy used up by this resistance is dissipated in useless thermal energy.
The task of maintaining a traveling wave distribution of current radiated by a linear antenna and minimizing the loss of energy is not dealt with by the following U.S. Patents, the disclosures of which, are specifically incorporated herein by reference:
U.S. Pat. No. 3,875,572 issued to Kay; PA0 U.S. Pat. No. 3,952,310 issued to Griffee et al.; PA0 U.S. Pat. No. 4,423,423 issued to Bush; and PA0 U.S. Pat. No. 4,629,978 issued to Aslan.
The patents identified above relate to antenna devices including dipole apparatus. In particular, the Kay patent describes a broad band antenna having a folded dipole including first and second open center portions, and transmission line feed points are located at the opposite sides of the second open center portion. The transformer element electrically lengthens the antenna into a half-wave folded dipole antenna for the reception of low-band signals, while simultaneously electrically opening the antenna into a full-wave dipole antenna for high-band reception.
The Griffee et al. patent is directed to a crossed dipole antenna apparatus configured with the ends of the adjacent dipoles connected together to form a version of folded dipole antenna wherein there are slot antennas between adjacent dipole antennas. The combination slot and dipole antennas provides broadband usable frequency range with acceptable radiation patterns.
The Bush patent relates to a folded dipole antenna comprising two conducting wires with each wire folded back over itself in a spaced parallel relationship. The ends of the folded wires are arranged so that like ends oppose each other. A load balancing means joins one set of the opposing ends of the folded conducting wires, and a load matching means connects the remaining set of opposing ends through the matching means to a radio frequency generator.
The Aslan patent describes a dipole antenna device comprising three mutually orthogonal antenna assemblies. Each antenna assembly includes an array of resistive thermocouples extending along a longitudinal axis. Within each assembly, conductive elements of discrete length extend transverse to their respective array, and are connected between each resistive thermocouple. The spacing between the conductive elements is approximately one-half wavelength of the mid-frequency of the range for which the antenna is designed.
Although these patents relate to antenna devices with dipole apparatus, they do not describe a dipole antenna where the orthogonal folded dipoles each consist of five segments designed so that the current have comparable magnitudes and are in phase quadrature to provide near hemispherical coverage.
While the above-cited references are instructive, a need remains to produce a dual polarized antenna that provides near hemispherical coverage and is driven from a single input. The present invention is intended to satisfy that need.